Syntax
------

INSERT DELAYED ...

Description
-----------

The DELAYED option for the INSERT statement is a MariaDB/MySQL extension to
standard SQL that is very useful if you have clients that cannot or need not
wait for the INSERT to complete. This is a common situation when you use
MariaDB for logging and you also periodically run SELECT and UPDATE statements
that take a long time to complete.

When a client uses INSERT DELAYED, it gets an okay from the server at once,
and the row is queued to be inserted when the table is not in use by any other
thread.

Another major benefit of using INSERT DELAYED is that inserts from many
clients are bundled together and written in one block. This is much faster
than performing many separate inserts.

Note that INSERT DELAYED is slower than a normal INSERT if the table is not
otherwise in use. There is also the additional overhead for the server to
handle a separate thread for each table for which there are delayed rows. This
means that you should use INSERT DELAYED only when you are really sure that
you need it.

The queued rows are held only in memory until they are inserted into the
table. This means that if you terminate mysqld forcibly (for example, with
kill -9) or if mysqld dies unexpectedly, any queued rows that have not been
written to disk are lost.

The number of concurrent INSERT DELAYED threads is limited by the
max_delayed_threads server system variables. If it is set to 0, INSERT DELAYED
is disabled. The session value can be equal to the global value, or 0 to
disable this statement for the current session. If this limit has been
reached, the DELAYED clause will be silently ignore for subsequent statements
(no error will be produced).

Limitations
-----------

There are some limitations on the use of DELAYED:

* INSERT DELAYED works only with MyISAM, MEMORY, ARCHIVE,
 and BLACKHOLE tables. If you execute INSERT DELAYED with another storage
engine, you will get an error like this: ERROR 1616 (HY000): DELAYED option
not supported for table 'tab_name'
* For MyISAM tables, if there are no free blocks in the middle of the data
 file, concurrent SELECT and INSERT statements are supported. Under these
 circumstances, you very seldom need to use INSERT DELAYED
 with MyISAM.
* INSERT DELAYED should be used only for
 INSERT statements that specify value lists. The server
 ignores DELAYED for INSERT ... SELECT
 or INSERT ... ON DUPLICATE KEY UPDATE statements.
* Because the INSERT DELAYED statement returns immediately,
 before the rows are inserted, you cannot use
 LAST_INSERT_ID() to get the
 AUTO_INCREMENT value that the statement might generate.
* DELAYED rows are not visible to SELECT
 statements until they actually have been inserted.
* After INSERT DELAYED, ROW_COUNT() returns the number of the rows you tried
to insert, not the number of the successful writes.
* DELAYED is ignored on slave replication servers, so that 
 INSERT DELAYED is treated as a normal
 INSERT on slaves. This is because
 DELAYED could cause the slave to have different data than
 the master. INSERT DELAYED statements are not safe for replication.
* Pending INSERT DELAYED statements are lost if a table is
 write locked and ALTER TABLE is used to modify the table structure.
* INSERT DELAYED is not supported for views. If you try, you will get an error
like this: ERROR 1347 (HY000): 'view_name' is not BASE TABLE
* INSERT DELAYED is not supported for partitioned tables.
* INSERT DELAYED is not supported within stored programs.
* INSERT DELAYED does not work with triggers.
* INSERT DELAYED does not work if there is a check constraint in place.
* INSERT DELAYED does not work if skip-new mode is active.

URL: https://mariadb.com/kb/en/insert-delayed/